Submerged Aerobic Fixed Film Reactor
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ACSION SAFF (Submerged Aerobic Fixed Film Reactor )
In ACSION SAFF processes, the microorganisms responsible for the conversation of organic material nutrients are attached to an inter packing material and nutrients are removed from the waste water flowing past the attached growth also known as a biofilm liquid layer.
The most common aerobic attached growth process used is the trickling filter in which waste water is distributed over the top area of a vessel containing non submerged packing material. Historically, rock was used most commonly as the packing material for trickling filters, with typical depths ranging from 1.25 to 2 m (4 to 6 ft). Most modern trickling filters vary in height from 5 to 10 m (16 to 33 ft) and are packed with a plastic packing material for biofilm attachment. The plastic packing material is designed such that about 90 to 95 percent of the volume in the tower consists of void space. Air circulation in the void space, by either natural draft or blowers, provides oxygen for the microorganisms growing as an attachment biofilm. Influent wastewater is distributed over the packing and flows as a non uniform liquid film over the attached biofilm. Excess biomass sloughs from the attached growth periodically and clarification is required for liquid/solids separation to provide an effluent with an acceptable suspended solids concentration. The solids are collected at the bottom of the clarifier and removed for waste-water sludge processing.
BIOLOGICAL NITRIFICATION
Nitrification is term used to describe the two-step biological process in which ammonia (nh4 –n) is oxidized to nitrite (no2-n) nitrite is oxidized to nitrite (no3-n). the need for nitrification in wastewater treatment arises from water quality cancers over (1) the effect of ammonia on receiving water with respect to do concentrations and fish toxicity, (2) the need to provide nitrogen removal to control eutrophication, and (3) the need to provide nitrogen control for water-reuse applications including groundwater recharge. For reference, the current (2001) drinking water maximum contaminant level (MCL) for nitrate nitrogen is 45 mg/l as nitrate or 10mg/l as nitrogen. the total concentration of organic and ammonia nitrogen concentration in municipal wastewater is typically in the range from 25 to 45 mg/l as nitrogen based on a flow rate of 450l/capited.d (120gal/capita.d). In many parts of the world with limited water supplies, total concentrations in excess of 200 mg/l as and have been measured in domestic wastewater.
PROCESS DESCRIPTION
As with BOD removal, nitrification can be accomplished in both suspended growth and attached growth biological processes. For suspended growth processes, a more common approach is to achieve nitrification along with BOD removal in the same single-sludge process, consisting of an aeration tank, clarifier, and sludge recycle system. In case where there is a significant potential for toxic and inhibitory substances in the wastewater, a two-sludge suspended growth system may be considered fibers in series with the first aeration tank/clarifier unit operated at a short SRT for BOD removal. The BOD and toxic substances are removed in the first unit, so that nitrification can proceed unhindered in the second. A portion of influent wastewater usually has to be bypassed to the second sludge system to provide a sufficient amount of solids for efficient solids flocculation and clarification.
Because the bacteria responsible for nitrification grow much more slowly than heterotrophic bacteria, systems designed for nitrification generally have much longer hydraulic and solids retention times than those for systems designed only for BOD removal.
In SAFF systems used for nitrification, most of the BOD must be removed before nitrifying organisms can be established. The heterotrophic bacteria have a higher biomass yield and thus can establish. The heterotrophic bacteria have a higher biomass yield and thus can dominate the surface area of fixed-film systems over nitrifying bacteria. Nitrification is accomplished in an attached growth reactor BOD removal or in a separate attached growth systems designed specifically for nitrification.
The placement of packing material in the aeration tank of the activated-sludge process dates back to the 1940s with the Hays and Griffith processes (WEF, 2000). Present day designs use more engineered packing and include the use of packing materials tat are suspended in the aeration tank with mixed liquor, fixed packing material placed in portions of the aeration tank, as well as submerged RBC. The advantages claimed for these activated sludge process enhancements are as follows :
Increased treatment capacityGreater process stabilityReduced sludge productionEnhanced sludge settles abilityReduced solids loadings on the secondary clarifierNo increase in operation and maintenance costs
MICROBIOLOGY
Aerobic autotrophic bacteria are responsible for nitrification in activated sludge and biofilm processes. Nitrification, as noted above is a two step process involving two groups of bacteria. In the first stage, ammonia is oxidized to nitrite by one group of autographic bacteria. In the second stage, nitrate is oxidized to nitrate by another group of autotrophic bacteria. It should be noted that the two groups of autographic bacteria are distinctly different. Starting with classical experiments on nitrification by Winogradsky (1891), the bacteria genera commonly noted for nitrification in wastewater treatment are the autotrophic bacteria Nitrosamines and Nitrobacter, which oxidize ammonia to nitrate and then to nitrate, respectively. Other autotrophic bacteria where were identified as being capable of oxidation ammonia to nitrate (prefix with Nitroso-) are Nitrosococcus, Nitrosospira, Nitrosolobus, and nitrosorobrio (printer, 1970). It should be noted that during the 1990, many more autotrophic bacteria were identified as being capable of oxidizing ammonia.
Besides Nitrobacter, nitrite can also be oxidized by other autotrophic bacteria (nitro-) genera: Nitrosococcus, Nitrosospira, Nitrospina and Nitroeystis. Using oligonucleotied probes for ammonia oxidizing bacteria, Wagner et al. (1995) showed that nitrosomonas was common in activated-sludge systems. For nitrate oxidation in activated sludge, Teske et al. (1994) found that Nitrococcus was quite prevalent. Whether different growth conditions can select for different genera of nitrifying bacteria or if their nitrification kinetics are significantly different is unknown at present. ACSION has developed this technique in a most modern way of versatile design and has supplied many such plants to meet the requirement of our valued customers.
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